some things to consider is the evaportor only because alot of your older systems don't have ac filters and airborne debris like lint, hair and other contaiminates gets into the evap. case. Which would cause poor ac performance. Also a cracked evaporator case would case poor ac performance because it will reduce the cooling capiciaty. Excessive engine heat also through the firewall can cause poor ac performance. There really isn't any way to proper clean a evaporator without removing it out of the case.

Another thing to consider is the dryier. The drier is like a filter system for your a/c. It carries moisture aborbing material at the bottom of the reciever or drier. (Same function) It basically removes any moisture in the lines and prevents it from entering the compressor which can damage the compressor. Thats basically the job of the drier. If the drier was clogged though it would cause ice or frost to build up at the outlet.